This application claims the priority of German Patent Application No. 197 47 222.2, filed Oct. 25, 1997, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to an internal-combustion engine system as well as to a process for the operation of such a system. Systems of this type, which are particularly used in motor vehicles, contain an emission control device that comprises a nitrogen oxide (NO.sub.x) storage catalyst and possible additional exhaust-gas-purifying components for minimizing nitrogen oxide emission.
In engine operating phases with an increased formation of nitrogen oxide, for example in the case of a lean engine air ratio (i.e., an air/fuel ratio above the stoichiometric value), nitrogen oxide is intermediately stored in the catalyst, for example, by means of an adsorption process. In a later phase which is suitable for this purpose, (e.g., at a rich air/fuel ratio-below the stoichiometric value) the stored nitrogen oxide is released (for example, by means of a corresponding desorption process) and converted by a subsequent reduction to nitrogen. The reduction reaction can take place in this catalyst or in another catalyst situated in the exhaust gas train or while exhaust gas is returned during the desorption phases into the combustion spaces of the engine.
The known difficulty of such systems is that, in the lean operation of the engine, sulfur dioxide is contained in the exhaust gas which, because of a sulfate formation, may result in a sulfur poisoning of the NOx storage catalyst. (The sulfur may originate from the fuel and/or from the engine oil.) Sulfur poisoning decreases the NO.sub.x storage capacity of the catalyst.
In W. Strehlau et al., "New Developments in Lean NO.sub.x Catalysis for Gasoline Fueled Passenger Cars in Europe", SAE 96 2047 (1996), the possibility of desulfurization of NO.sub.x storage catalysts sulfur-aged in this manner is described; that is, freeing catalysts from the poisoning sulfate. In this case, suitable conditions are (1) increased exhaust gas temperatures of above 600.degree. C., and (2) rich exhaust gas compositions, i.e, a ratio of oxygen or air to fuel or unburnt hydrocarbons in the exhaust gas is guided through the catalyst below the stoichiometric value. This ratio is called the catalyst air ratio.
European Published Patent Application EP 0 636 770 A1, discloses an internal-combustion engine system in which an existing NO.sub.x storage catalyst is periodically subjected to desulfurization phases. The necessity of a respective desulfurization phase is determined from a detection of the duration of a respective NO.sub.x regeneration phase (a desorption phase of the catalyst), which is determined by means of the measuring signal of a lambda probe arranged downstream of the NO.sub.x storage catalyst. The lambda probe measures the air/fuel ratio at this point and thus the catalyst air ratio on the output side of the catalyst. This ratio decreases as soon as no more nitrogen oxide is released in a desorption phase because then the unburnt hydrocarbons contained in the exhaust gas are no longer consumed as reducing agents for the conversion of desorbed nitrogen oxide.
In the case of a constant operating point, the regeneration duration is proportional to the stored NO.sub.x mass so that a direct conclusion can be drawn concerning the stored NO.sub.x mass. The NO.sub.x mass stored in the lean phase can be calculated, for example, by means of a catalyst model and, from it, the pertaining theoretical regeneration duration can be calculated. If the measured regeneration duration is shorter than the calculated one, a conclusion can be drawn concerning a reduced NO.sub.x storage capacity of the catalyst because of sulfur poisoning. Therefore, a desulfurization phase may be required. As an alternative, the sulfur aging of the NO.sub.x storage catalyst can also be determined by means of an NO.sub.x sensor downstream of the catalyst. If the NO.sub.x concentration rises very rapidly downstream of the catalyst during an adsorption phase, this indicates a reduced NO.sub.x storage capacity.
For implementing desulfurization, it is suggested in EP 0 636 770 A1 to change the internal-combustion engine from a lean to a rich engine air ratio and, as required, additionally activate an electric heating device that is assigned to the NO.sub.x storage catalyst for this purpose. It is known that the engine exhaust gas temperature is lower when a rich air/fuel mixture is supplied to the engine. The desulfurization phase is maintained for approximately 10 minutes.
In Published German Patent Application DE 195 22 165 A1, for implementing the desulfurization phases in an internal-combustion engine system, a rich engine air ratio and a later ignition point is adjusted for the respective engine cylinder. In addition, secondary air is supplied to the exhaust gas upstream of the NO.sub.x storage catalyst. The desulfurization phases are controlled by an engine control device, preferably in such a manner that the catalyst temperature sensed by a corresponding temperature sensor is controlled to a desired increased value by means of a suitable adjustment of the engine air ratio and of the ignition point. For this purpose, some of the cylinders are operated in a rich manner and the other cylinders are operated in a lean manner, with the intake air quantity appropriately increased for the cylinders with the rich combustion operation. In this case, a lowering of the engine power is prevented. The desulfurization phase is in each case maintained for a definable time period.
The object of the present invention is to provide an internal-combustion engine system as well as an operating process therefor, in which periodic desulfurization of the NO.sub.x storage catalyst occurs in the running engine operation, while the engine operating condition is affected as little as possible.
In the case of a first embodiment of the internal-combustion engine system according to the present invention, the desulfurization devices comprise devices for feeding secondary air into the NO.sub.x storage catalyst. The oxygen contained in the secondary air reacts exothermally with a portion of the reducing agent contained in the exhaust gas, such as unburnt hydrocarbons, hydrogen and carbon monoxide. As a result, the catalyst can be brought to the increased temperature which is optimal for the desulfurization (sulfate regeneration) without the requirement (1) of an additional heating from the outside by means of an electric heating device or the like, or (2) that the engine itself must provide correspondingly high exhaust gas temperatures. Furthermore, the internal-combustion engine system contains a lambda probe for detecting the catalyst air ratio which is arranged downstream of the NO.sub.x storage catalyst. Correspondingly, during the desulfurization phases, the internal-combustion engine is operated with a rich engine air ratio in order to provide a sufficient quantity of reducing agents in the exhaust gas, and simultaneously secondary air is metered into the NO.sub.x storage catalyst. In this case, the information of the lambda probe arranged downstream of the NO.sub.x catalyst is used for adjusting the engine air ratio and the secondary air feeding rate.
In a second embodiment of the present invention, a temperature sensor is provided for detecting the temperature of the NO.sub.x storage catalyst or of the exhaust gas guided through it. This temperature information can be used to adjust the engine air ratio and/or the secondary air feeding rate (and therefore the catalyst air ratio) in a manner which differs according to a specific requirement; for example: (1) as a purely protective measure with respect to overheating the catalyst, or (2) for adjusting to a definable desired temperature value.
In a process according to the present invention, the continuous monitoring of the actual value of the catalyst air ratio by the lambda probe downstream of the NO.sub.x catalyst can be utilized to adjust the catalyst air ratio by the variable adjustment of the engine air ratio and/or of the secondary air feeding rate to a definable desired value that is optimal for the desulfurization of the catalyst.
Another embodiment of the system according to the present invention contains an electrically controllable throttle valve in the engine intake system that can be controlled by the desulfurization control which may, for example, be integrated in a central engine control unit. This desulfurization control is utilized for variably adjusting the throttle valve during the desulfurization phases such that the internal-combustion engine generates a desired torque independently of the course of the desulfurization operation. In this manner the power output of the engine, and thus when the engine is used in a motor vehicle the driving condition of the vehicle, remains unaffected by the presence and course of a respective desulfurization phase.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.